1.
When water converts to vapor phase from liquid phase, there is
no any temperature change. In other words, when a substance changes its phase,
the temperature remains constant.
So we can use the formula
Q = mL
Where Q is the heat (J), m is the mass
(g) and L is the latent heat (J/g).
M = 500 g
L = Latent heat of vaporization = 2230 J/g
Hence, Q = 500 g x 2230 J/g = 1115000 J
= 1115 kJ
Hence, 1115 kJ of heat is needed to vaporize 500 g of water at its boiling point.
2.
Let’s assume that water is at its freezing point (0 ⁰C). Then
there is only phase conversion and no any temperature change. Again we can use
the formula,
Q = mL
Here, Q is the heat
that given off (J), m is the mass (g) and L is the latent heat of fusion (J/g).
Q = 5100 J
L = 334 J/g
Hence, 5100 J = m x
334 J/g
m = 5100 J / 334 J/g
m = 15.269 g
Hence, the mass of water is 15.269 g.
3. When condensing vapor into its liquid again it
is only a phase conversion and temperature remains as constant (100 ⁰C).
Hence, we can use the formula,
Q = mL
Here, Q is the heat
that given off (J), m is the mass of vapor (g) and L is the Latent heat of
vaporization (J/g).
Q = 57.500
J
L = 2230 J/g
By applying the
equation,
57.500 J = m x 2,230 J/g
m =0.0258 g
Hence, the mass of steam is 0.0258 g.
